1. Field of the Invention
This invention relates generally to an apparatus for detecting ground faults on a distribution system, and more specifically, to such an apparatus having a variable impedance depending upon load current.
2. Description of the Prior Art
Electrical transmission and distribution lines, station transformers, buses, breakers, motors, and power generators must be protected against insulation deterioration and consequent short circuits that can cause collapse of the power system and serious and expensive apparatus damage. In addition, fault conditions can be caused by lightning or wind forcing trees or branches to contact, and sometimes break, distribution conductors. These broken-conductor faults may cause extended service interruptions and therefore must be detected and repaired.
Most electric utility distribution systems use three-phase four-wire grounded-neutral circuits to supply both three-phase and single-phase-to-neutral loads. These circuits are protected against faults by three-phase circuit breakers at the substation, generally equipped with phase-overcurrent and ground-overcurrent (i.e., zero-sequence or 3I.sub.O) relays. Ground-overcurrent relay trip settings must allow for normal circuit load unbalances, coordinate with other protective devices, and prevent tripping for large unbalances among each phase-to-neutral load on large in-rush currents during cold-load pickup and normal circuit switching. In some distribution circuits, the unbalanced loads could require a ground relay trip setting as high, or higher than one-half of the phase relay setting. As a result, high-impedance single-phase-to-ground faults, characterized by low fault-current magnitudes, are not generally detectable by overcurrent protective devices commonly used on distribution circuits, because such faults do not produce sufficient current or circuit unbalance during light loading to operate the phase-overcurrent or ground-overcurrent protective relays. Such high-impedance faults may result from the breaking of a conductor that hangs free or contacts a high-impedance surface such as a tree, antenna, dry grass, brush, gravel, concrete, or asphalt. These faults, although not so destructive to the distribution system, may cause extended service interruption and brush fires.
The following U.S. patents, all assigned to the assignee of the present invention, disclose novel apparatus for detecting high-impedance or broken-conductor ground faults on four-wire electrical power distribution systems: U.S. Pat. Nos. 4,297,740, 4,347,542; 4,357,644; and 4,363,066. The apparatus of U.S. Pat. No. 4,297,740 evaluates the zero sequence current on the distribution system to detect a high-impedance fault. The amount of zero sequence current necessary to trip the protective relay is determined by the phase currents or the positive sequence current. U.S. Pat. No. 4,357,644 discloses an electromechanial apparatus to detect high-impedance faults. The operating quantity for this electromechanical apparatus is the zero sequence current. The restraint quantity is developed using a function of any one of the following signals: any two-phase current, the positive sequence current minus the zero sequence current, the three-phase currents, or the positive sequence current. U.S. Pat. No. 4,347,542 discloses an improvement to U.S. Pat. No. 4,357,644 to improve the equality of relay response for a given type of fault for all phase combinations. This improved response is accomplished by modifying the winding arrangement on the magnetic core of the relay. The improved embodiment utilizes the same quantities to form the operating and restraining signals. In U.S. Pat. No. 4,363,066, the operating quantity is a function of the zero sequence current and the restraining quantity is a function of the pre-fault phase current less the zero sequence current. U.S. patent application Ser. No. 528,354 (filed Sept. 1, 1983 and assigned to the assignee of the present invention) also discloses a protective relay apparatus for detecting high-impedance ground faults. This protective relay, intended for three-wire distribution systems, examines the ratio of the positive and negative sequence currents flowing on the distribution system and indicates the occurrence of a fault when the ratio exceeds a predetermined value.
Although the protective relays disclosed in the above-mentioned U.S. patents and application operate satisfactorily, their use results in certain disadvantages related to the cost of mounting and wiring this additional relay in the distribution fault protection circuitry, and coordinating the ground-fault relay with the relays for detecting broken phase conductor faults. The present invention, a relay employing variable impedance, is designed to overcome these disadvantages.
U.S. Pat. No. 2,619,522 discloses a protective relay having a variable operating characteristic depending upon the magnitude of a quantity in the circuit with which it is associated. This protective relay employs a typical operating winding, and restraining windings energized from the operating winding via a saturable transformer. By including an auxiliary overcurrent relay in the circuit of the saturable transformer, the normal operating characteristics of the relay are modified so that at higher current levels, operation is essentially instantaneous.